Polymer electrolyte with aromatic sulfone crosslinking

ABSTRACT

A method is provided for obtaining crosslinked polymers having pendent sulfonic acid groups by crosslinking through the sulfonic acid groups or their precursors with aromatic crosslinkers or aromatic pendent crosslinking groups to form aromatic sulfones. Such crosslinked polymers may be used to make polymer electrolyte membranes (PEM&#39;s) that may be used in electrolytic cells such as fuel cells.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. Ser. No. 11/278,459, filed Apr.3, 2006, now U.S. Pat. No. 7,847,035, which is a divisional of U.S. Ser.No. 10/720,906, filed Nov. 24, 2003, issued as U.S. Pat. No. 7,060,756,the disclosure of which is herein incorporated by reference.

FIELD OF THE INVENTION

This invention relates to a method of obtaining crosslinked polymershaving pendent sulfonic acid groups by crosslinking through the sulfonicacid groups or their precursors with aromatic crosslinkers or aromaticpendent crosslinking groups to form aromatic sulfones. Such crosslinkedpolymers may be used to make polymer electrolyte membranes (PEM's) thatmay be used in electrolytic cells such as fuel cells.

BACKGROUND OF THE INVENTION

Copolymers of tetrafluoroethylene (TFE) and a co-monomer according tothe formula: FSO₂—CF₂—CF₂—O—CF(CF₃)—CF₂—O—CF═CF₂ are known and sold insulfonic acid form, i.e., with the FSO₂— end group hydrolyzed to HSO₃—,under the trade name Nafion® by DuPont Chemical Company, Wilmington,Del. Nafion® is commonly used in making polymer electrolyte membranesfor use in fuel cells.

Copolymers of tetrafluoroethylene (TFE) and a co-monomer according tothe formula: FSO₂—CF₂—CF₂—O—CF═CF₂ are known and used in sulfonic acidform, i.e., with the FSO₂— end group hydrolyzed to HSO₃—, in makingpolymer electrolyte membranes for use in fuel cells.

U.S. patent application Ser. No. 10/325,278, filed Dec. 19, 2002, thedisclosure of which is incorporated herein by reference, discloses apolymer electrolyte membrane having a thickness of 90 microns or lessand comprising a polymer, said polymer comprising a highly fluorinatedbackbone and recurring pendant groups according to the formula:YOSO₂—CF₂—CF₂—CF₂—CF₂—O-[polymer backbone]where Y is H⁺ or a monovalent cation such as an alkali metal cation.Typically, the membrane is a cast membrane. Typically, the polymer has ahydration product of greater than 22,000. Typically, the polymer has anequivalent weight of 800-1200.

International Patent Application Publication No. WO 01/27167 purportedlydiscloses a crosslinked fluorocarbon polymeric composition havinghydrophilic functional groups which is crosslinked with fluorinatedcrosslinking groups.

U.S. Patent Application Publication No. 2003/0032739 discloses acovalently crosslinked polymer or polymer membrane consisting of one ormore polymers, which may bear precursors of cation exchange groups,which are crosslinked through the reaction of sulfinate groups —SO₂Me onthe polymer with crosslinkers which may include halo aromatics to formcross-linking bridges which may include:polymer-SO₂-arylene-SO₂-polymer.

U.S. Pat. No. 6,090,895 discloses a method for making crosslinked acidicpolymers useful as ion conductive membranes, such as crosslinkedsulfonated polyether ketones, sulfonated polysulfones, sulfonatedpolystyrenes, and other acidic polymers, by crosslinking with a specieswhich generates an acidic functionality. The crosslinker preferablybinds to acid functions by conversion of acid groups to imidefunctionality, which, due to the acidity of the N—H bonds therein,compensate for the acidity lost by the occupation of the acid groups andthus preserve membrane conductivity while contributing to membranestrength and resistance to swelling.

U.S. Patent Application Publication No. 2003/0092940 discloses a methodfor making aromatic-imide and aromatic-methylidynetrissulfonyl speciesby reaction of aromatic species with a reactant according to theformula:(X—SO₂—)_(m)-QH—(—SO₂—R₁)_(n)wherein Q is C or N; wherein each X is independently selected from thegroup consisting of halogens, typically F or Cl; wherein each R₁ isindependently selected from the group consisting of aliphatic andaromatic groups, which may or may not be straight-chain, branched,cyclic, heteroatomic, polymeric, halogenated, fluorinated orsubstituted; wherein m is greater than 0; wherein m+n=2 when Q is N; andwherein m+n=3 when Q is C. Ar may be derived from an aromatic polymericcompound. In addition, the reference discloses compounds according tothe formula: (Ar—SO₂—)_(m)-QH—(—SO₂—R₁)_(n) wherein R₁ comprises ahighly acidic group selected from sulfonic acid, carboxylic acid andphosphonic acid, and Ar is derived from an aromatic compound.

SUMMARY OF THE INVENTION

The present invention provides crosslinked polymers and method of makingcrosslinked polymers by a comprising the steps of: a) providing a highlyfluorinated polymer comprising pendent groups which include a groupaccording to the formula —SO₂X, wherein each X is independently selectedfrom F, Cl, Br, I, —OH or —O—SO₂R² wherein R² is an aliphatic groupcontaining 1-18 carbon atoms which may be substituted; and b) reactingthe polymer with a crosslinking agent according to the formula Ar_(n)R¹,wherein each Ar is selected independently from aromatic groupscontaining 6-24 carbon or nitrogen atoms and wherein each Ar may besubstituted, wherein R¹ is a direct bond or an aromatic or aliphaticlinking group, wherein R¹ may be straight-chain, branched, cyclic,heteroatomic, polymeric, halogenated, fluorinated or substituted, andwhere n is at least 2, to form crosslinks comprising units according tothe formula (—SO₂Ar)_(n)R¹. In one embodiment, the polymer comprisespendent groups that include —SO₂F and at least a portion of the —SO₂Fgroups are then converted to —SO₂Cl or —SO₂—O—SO₂R² for reaction. In oneembodiment, the polymer is formed into a membrane prior to crosslinking,typically one having a thickness of 90 microns or less. Typically, theremaining —SO₂X groups are converted to sulfonic acid groups aftercrosslinking.

In another aspect, the present invention provides a highly fluorinatedcrosslinked polymer comprising: a backbone, pendent groups whichcomprise sulfonic acid groups, and crosslinks comprising units accordingto the formula (—SO₂Ar)_(n)R¹ wherein each Ar is selected independentlyfrom aromatic groups containing 6-24 carbon or nitrogen atoms andwherein each Ar may be substituted, wherein R¹ is a direct bond or anaromatic or aliphatic linking group, wherein R may be straight-chain,branched, cyclic, heteroatomic, polymeric, halogenated, fluorinated orsubstituted, and where n is at least 2. In one embodiment, the polymeris a polymer electrolyte membrane, typically having a thickness of 90microns or less. Typical pendent groups include groups according to theformula —O—(CF₂)₄—SO₃H and groups according to the formula—O—CF₂—CF(CF₃)—O—CF₂—CF₂—SO₃H.

In another aspect, the present invention provides a method of making acrosslinked polymer comprising the steps of: a) providing a highlyfluorinated polymer comprising first pendent groups which include agroup according to the formula —SO₂X, wherein each X is independentlyselected from F, Cl, Br, I, —OH or —O—SO₂R² wherein R² is an aliphaticgroup containing 1-18 carbon atoms which may be substituted, and secondpendent groups which include groups —Ar, wherein each Ar is selectedindependently from aromatic groups containing 6-24 carbon or nitrogenatoms and wherein each Ar may be substituted; and b) reacting thepolymer to form crosslinks between the first and second pendent groupscomprising units according to the formula —SO₂Ar—. In one embodiment,the polymer comprises first pendent groups that include —SO₂F and atleast a portion of the —SO₂F groups are then converted to —SO₂Cl or—SO₂—O—SO₂R² for reaction. In one embodiment, the polymer is formed intoa membrane prior to crosslinking, typically one having a thickness of 90microns or less. Typically, the remaining —SO₂X groups are converted tosulfonic acid groups after crosslinking.

In another aspect, the present invention provides a highly fluorinatedcrosslinked polymer comprising: a backbone, pendent groups whichcomprise sulfonic acid groups, and crosslinks comprising units accordingto the formula —SO₂Ar— wherein each Ar is selected independently fromaromatic groups containing 6-24 carbon or nitrogen atoms and whereineach Ar may be substituted. In one embodiment, the polymer is a polymerelectrolyte membrane, typically having a thickness of 90 microns orless. Typical pendent groups include groups according to the formula—O—(CF₂)₄—SO₃H and groups according to the formula—O—CF₂—CF(CF₃)—O—CF₂—CF₂—SO₃H.

In this application:

“equivalent weight” (EW) of a polymer means the weight of polymer whichwill neutralize one equivalent of base;

“hydration product” (HP) of a polymer means the number of equivalents(moles) of water absorbed by a membrane per equivalent of sulfonic acidgroups present in the membrane multiplied by the equivalent weight ofthe polymer; and

“highly fluorinated” means containing fluorine in an amount of 40 wt %or more, typically 50 wt % or more and more typically 60 wt % or more;and

“substituted” means, for a chemical species, substituted by conventionalsubstituents which do not interfere with the desired product or process,e.g., substituents can be alkyl, alkoxy, aryl, phenyl, halo (F, Cl, Br,I), cyano, nitro, etc.

DETAILED DESCRIPTION

Briefly, the present invention provides a method of obtainingcrosslinked polymers having pendent sulfonic acid groups by crosslinkingthrough the sulfonic acid groups or their precursors with aromaticcrosslinkers or aromatic pendent crosslinking groups to form aromaticsulfones. Such crosslinked polymers may be used to make polymerelectrolyte membranes (PEM's) that may be used in electrolytic cellssuch as fuel cells.

PEM's manufactured from the crosslinked polymer according to the presentinvention may be used in the fabrication of membrane electrodeassemblies (MEA's) for use in fuel cells. An MEA is the central elementof a proton exchange membrane fuel cell, such as a hydrogen fuel cell.Fuel cells are electrochemical cells which produce usable electricity bythe catalyzed combination of a fuel such as hydrogen and an oxidant suchas oxygen. Typical MEA's comprise a polymer electrolyte membrane (PEM)(also known as an ion conductive membrane (ICM)), which functions as asolid electrolyte. One face of the PEM is in contact with an anodeelectrode layer and the opposite face is in contact with a cathodeelectrode layer. Each electrode layer includes electrochemicalcatalysts, typically including platinum metal. Gas diffusion layers(GDL's) facilitate gas transport to and from the anode and cathodeelectrode materials and conduct electrical current. The GDL may also becalled a fluid transport layer (FTL) or a diffuser/current collector(DCC). The anode and cathode electrode layers may be applied to GDL's inthe form of a catalyst ink, and the resulting coated GDL's sandwichedwith a PEM to form a five-layer MEA. Alternately, the anode and cathodeelectrode layers may be applied to opposite sides of the PEM in the formof a catalyst ink, and the resulting catalyst-coated membrane (CCM)sandwiched with two GDL's to form a five-layer MEA. The five layers of afive-layer MEA are, in order: anode GDL, anode electrode layer, PEM,cathode electrode layer, and cathode GDL. In a typical PEM fuel cell,protons are formed at the anode via hydrogen oxidation and transportedacross the PEM to the cathode to react with oxygen, causing electricalcurrent to flow in an external circuit connecting the electrodes. ThePEM forms a durable, non-porous, electrically non-conductive mechanicalbarrier between the reactant gases, yet it also passes H⁺ ions readily.

The polymer to be crosslinked comprises a backbone, which may bebranched or unbranched but is typically unbranched. The backbone isfluorinated, typically highly fluorinated, and more typicallyperfluorinated. The backbone may comprise units derived fromtetrafluoroethylene (TFE), i.e., typically —CF₂—CF₂— units, and unitsderived from co-monomers, typically including at least one according tothe formula CF₂═CY—R¹⁰ where Y is typically F but may also be CF₃, andwhere R¹⁰ is a first pendant group which includes a group according tothe formula —SO₂X wherein X is selected from F, Cl, Br, I, —OH or—O—SO₂R² wherein R² is an aliphatic group containing 1-18 carbon atomswhich may be substituted. Where —SO₂X is a sulfonyl halide, X is mosttypically F. In an alternative embodiment, first pendant groups R¹⁰ maybe added to the backbone by grafting. Typically, first pendant groupsR¹⁰ are highly fluorinated and more typically perfluorinated. R¹⁰ may bearomatic or non-aromatic. Typically, R¹⁰ is —R¹¹—SO₂X, where R¹¹ is abranched or unbranched perfluoroalkyl or perfluoroether group comprising1-15 carbon atoms and 0-4 oxygen atoms. R¹¹ is typically —O—R¹²— whereinR¹² is a branched or unbranched perfluoroalkyl or perfluoroether groupcomprising 1-15 carbon atoms and 0-4 oxygen atoms. R¹¹ is more typically—O—R¹³— wherein R¹³ is a perfluoroalkyl group comprising 1-15 carbonatoms. Examples of R¹¹ include:

—(CF₂)_(n)— where n is 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14 or15

(—CF₂CF(CF₃)—)_(n) where n is 1, 2, 3, 4, or 5

(—CF(CF₃)CF₂—)_(n) where n is 1, 2, 3, 4, or 5(—CF₂CF(CF₃)—)_(n)—CF₂—where n is 1, 2, 3 or 4

(—O—CF₂CF₂—)_(n) where n is 1, 2, 3, 4, 5, 6 or 7

(—O—CF₂CF₂CF₂—)_(n) where n is 1, 2, 3, 4, or 5

(—O—CF₂CF₂CF₂CF₂—)_(n) where n is 1, 2 or 3

(—O—CF₂CF(CF₃)—)_(n) where n is 1, 2, 3, 4, or 5

(—O—CF₂CF(CF₂CF₃)—)_(n) where n is 1, 2 or 3

(—O—CF(CF₃)CF₂—)_(n) where n is 1, 2, 3, 4 or 5

(—O—CF(CF₂CF₃)CF₂—)_(n) where n is 1, 2 or 3

(—O—CF₂CF(CF₃)—)_(n)—O—CF₂CF₂— where n is 1, 2, 3 or 4

(—O—CF₂CF(CF₂CF₃)—)_(n)—O—CF₂CF₂— where n is 1, 2 or 3

(—O—CF(CF₃)CF₂—)_(n)—O—CF₂CF₂— where n is 1, 2, 3 or 4

(—O—CF(CF₂CF₃)CF₂—)_(n)—O—CF₂CF₂— where n is 1, 2 or 3

—O—(CF₂)_(n)— where n is 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13 or 14

R¹⁰ is typically —O—CF₂CF₂CF₂CF₂—SO₂X or —O—CF₂—CF(CF₃)—O—CF₂—CF₂—SO₂Xand most typically —O—CF₂CF₂CF₂CF₂—SO₂X. The —SO₂X group is mosttypically —SO₂F during polymerization, i.e., X is F. The —SO₂X group istypically converted to —SO₃H at some point prior to use of thefluoropolymer as an polymer electrolyte. The fluoromonomer providingfirst side group R¹⁰ may be synthesized by any suitable means, includingmethods disclosed in U.S. Pat. No. 6,624,328.

The polymer may be made by any suitable method, including emulsionpolymerization, extrusion polymerization, polymerization insupercritical carbon dioxide, solution or suspension polymerization, andthe like, including methods disclosed in U.S. patent application Ser.No. 10/697,768, filed Oct. 30, 2003 and references cited therein.

Where the —SO₂X group is —SO₂F during polymerization, some of the —SO₂Fgroups may be converted to more reactive groups prior to crosslinking,such as —SO₂Cl, —SO₂Br, —SO₂I or —O—SO₂R² wherein R² is an aliphaticgroup containing 1-18 carbon atoms which may be substituted, moretypically containing 1-8 carbon atoms, and most typically methyl orethyl. Typically, between 1 and 50% of —SO₂F groups are converted tomore reactive groups. —SO₂F groups may be converted by any suitablemethod. —SO₂F groups may be converted to —SO₂Cl groups by any suitablemethod. In one such method, —SO₂F groups are reduced to —SO₂H by use ofa suitable reducing agent, such as a hydrazine or mercaptan such asmercaptoethanol, and subsequently converted to —SO₂Cl with ahypochloride. In another such method, —SO₂F groups may be converted to—SO₂Cl groups by reaction with oxalyl chloride in dry toluene withpyridine catalyst. —SO₂F groups may be converted to —O—SO₂R² groups byany suitable method. In one such method, —SO₂F groups are converted byexchange with R²—SO₂—O—SO₂R², e.g. CH₃—SO₂—O—SO₂—CH₃. In another suchmethod, —SO₂F groups are converted by reaction with R²—SO₃H and P₂O₅.

In one embodiment of the present invention, the polymer additionallycomprises second pendent groups which include groups —Ar, wherein eachAr is selected independently from aromatic groups containing 6-24 carbonor nitrogen atoms and wherein each Ar may be substituted. Typical Argroups include phenyl, naphthyl, anthracyl, phenanthracyl, biphenyl,terphenyl, fluoryl, indyl, fluoranthyl, pyridyl, puryl and the like.When substituents are present, they are typically electron donatingsubstituents, such as alkoxy, hydroxy, amine, alkyl and the like. Thesecond pendent groups may be introduced into the polymer byter-polymerization with monomers such as CF₂═CY—R²⁰ where Y is typicallyF but may also be CF₃, and where R²⁰ is the second pendant group. In analternative embodiment, first pendant groups R²⁰ may be added to thebackbone by grafting. Second pendent groups R²⁰ may be according to theformula —R¹¹—Ar, where R¹¹ is as described above. In this embodiment ofthe invention, the polymer is crosslinked by joining first and secondpendent groups. Additional crosslinking agent, described below, may beadded but is unnecessary. The second pendent groups are present in thepolymer in a numerical (molar) amount that is less than the amount ofthe first pendent groups, typically less than 90% relative to the amountof the first pendent groups and more typically less than 50%.

In one embodiment of the present invention, the polymer is crosslinkedby reaction with a crosslinking agent according to the formula Ar_(n)R¹,wherein Ar is as described above, wherein R¹ is a direct bond or anaromatic or aliphatic linking group, wherein R¹ may be straight-chain,branched, cyclic, heteroatomic, polymeric, halogenated, fluorinated orsubstituted, and where n is at least 2. n is typically 2-4, moretypically 2-3, and most typically 2. R¹ typically contains 1-120 carbon,oxygen or nitrogen atoms, but may be larger if it is polymeric. R¹ istypically aliphatic. R¹ is more typically a straight-chain or branchedalkylene, alkoxy or polyether group containing 1-20 carbon or oxygenatoms. R¹ may also be a polymer or oligomer, especially where n is alarger number, e.g. greater than four. R¹ is typically fluorinated, moretypically highly fluorinated, and most typically perfluorinated. WhereR¹ is a direct bond, n must be 2 and the crosslinking agent is Ar—Ar,e.g., biphenyl. Typically, R¹ attaches to each Ar through an oxygenatom. Typically R¹ is —O—R³—O—, where R³ is an aliphatic linking groupcontaining 1-18 carbon or oxygen atoms, more typically containing 1-8carbon or oxygen atoms. Examples of crosslinking agents according to thepresent invention include: diphenyl ether, diphenoxy alkanes, diphenoxyethers, diphenoxy polyethers, and the like.

The crosslinking agent and polymer may be mixed by any suitable method,including mixing in solution or suspension, kneading, milling, or thelike. The amount of crosslinking agent mixed with the polymer istypically selected so that the resulting crosslinked polymer will meetthe hydration product and equivalent weight parameters described below.

In one embodiment of the present invention, the polymer orpolymer/crosslinking agent blend is formed into a membrane prior tocrosslinking Any suitable method of forming the membrane may be used.The polymer is typically cast from a suspension. Any suitable castingmethod may be used, including bar coating, spray coating, slit coating,brush coating, and the like. Alternately, the membrane may be formedfrom neat polymer in a melt process such as extrusion. After forming,the membrane may be annealed. Typically the membrane has a thickness of90 microns or less, more typically 60 microns or less, and mosttypically 30 microns or less. A thinner membrane may provide lessresistance to the passage of ions. In fuel cell use, this results incooler operation and greater output of usable energy. Thinner membranesmust be made of materials that maintain their structural integrity inuse.

The crosslinking reaction may be carried out by any suitable method.Typically, the reaction is accomplished by application of heat,typically to a temperature of 160° C. or more. Typically, a catalystsuch as a Lewis acid is introduced. The step of crosslinking the polymermay occur in whole or part during annealing of the membrane or may becarried out separate from any annealing step. During the crosslinkingstep, aromatic sulfone groups are formed according to the formula:—SO₂Ar—. Where a crosslinking agent is used, the resulting crosslinkscomprise units according to the formula (—SO₂Ar)_(n)R¹. Where first andsecond pendent groups join to form crosslinks, they comprise unitsaccording to the formula —SO₂Ar—.

After crosslinking, the remaining sulfur-containing functions of thependant groups may be converted to sulfonic acid form by any suitableprocess. Sulfonyl halide groups may be converted by hydrolysis. In onetypical process, the polymer is immersed in an aqueous solution of astrong base and subsequently acidified. In one typical embodiment, apolymer membrane is immersed in 15% KOH in water at 80° C. for 1 hour,then washed twice in 20% nitric acid at 80° C., then boiled in deionizedwater twice. Sulfonyl anhydride groups may be converted by hydrolysis,with removal of remaining R²—SO₃H.

The acid-functional pendant groups typically are present in an amountsufficient to result in an hydration product (HP) of greater than15,000, more typically greater than 18,000, more typically greater than22,000, and most typically greater than 25,000. In general, higher HPcorrelates with higher ionic conductance.

The acid-functional pendant groups typically are present in an amountsufficient to result in an equivalent weight (EW) of less than 1200,more typically less than 1100, and more typically less than 1000, andmore typically less than 900.

In a further embodiment, the polymer or polymer/crosslinking agent blendmay be imbibed into a porous supporting matrix prior to crosslinking,typically in the form of a thin membrane having a thickness of 90microns or less, more typically 60 microns or less, and most typically30 microns or less. Any suitable method of imbibing the polymer into thepores of the supporting matrix may be used, including overpressure,vacuum, wicking, immersion, and the like. The polymer becomes embeddedin the matrix upon reaction of the amidine groups. Any suitablesupporting matrix may be used. Typically the supporting matrix iselectrically non-conductive. Typically, the supporting matrix iscomposed of a fluoropolymer, which is more typically perfluorinated.Typical matrices include porous polytetrafluoroethylene (PTFE), such asbiaxially stretched PTFE webs.

It will be understood that membranes made according to the method of thepresent invention may differ in chemical structure from those made byother methods, in the structure of crosslinks, the placement ofcrosslinks, the placement of acid-functional groups, and the like.

This invention is useful in the manufacture of polymer electrolytemembranes for use in electrolytic cells such as fuel cells.

Various modifications and alterations of this invention will becomeapparent to those skilled in the art without departing from the scopeand principles of this invention, and it should be understood that thisinvention is not to be unduly limited to the illustrative embodimentsset forth hereinabove.

We claim:
 1. A highly fluorinated crosslinked polymer comprising: abackbone, pendent groups which comprise sulfonic acid groups, andcrosslinks comprising units according to the formula (—SO₂Ar)_(n)R¹wherein each Ar is selected independently from aromatic groupscontaining 6-24 carbon or nitrogen atoms and wherein each Ar may besubstituted, wherein R¹ is a direct bond or an aromatic or aliphaticlinking group, wherein R¹ may be straight-chain, branched, cyclic,heteroatomic, polymeric, halogenated, fluorinated or substituted, andwhere n is at least
 2. 2. The polymer according to claim 1 wherein eachAr is a phenylene group which may be substituted.
 3. The polymeraccording to claim 1 wherein one or more Ar is substituted with anelectron donating group.
 4. The polymer according to claim 1 wherein oneor more Ar is substituted with an alkoxy group.
 5. The polymer accordingto claim 1 wherein R¹ is an aliphatic linking group containing 1-20carbon or oxygen atoms.
 6. The polymer according to claim 1 wherein n is2.
 7. The polymer according to claim 1 wherein said pendent groups areaccording to the formula —O—(CF₂)₄—SO₃H.
 8. The polymer according toclaim 1 wherein said pendent groups are according to the formula—O—CF₂—CF(CF₃)—O—CF₂—CF₂—SO₃H.
 9. The polymer according to claim 1having an equivalent weight of less than
 1200. 10. A polymer electrolytemembrane comprising the highly fluorinated crosslinked polymer accordingto claim
 1. 11. The polymer electrolyte membrane according to claim 10having a thickness of 90 microns or less.
 12. The polymer electrolytemembrane according to claim 10 wherein said highly fluorinatedcrosslinked polymer is embedded in a porous supporting matrix.
 13. Thepolymer electrolyte membrane according to claim 12 wherein said poroussupporting matrix is a porous polytetrafluoroethylene web.